I think the reason you are contacted individually is for legal reasons. When contacted, the main reason being to check you are ok and if you need any assistance. The other is to check you understand the legal responsibilities that you are obliged to follow.
That, I believe is why individuals need to be contacted, and guardians in the case of under 18’s (or 16’s I’m MLT sure) as you cannot vouch for another adult.
The call takes around 4 minutes; not a massive problem if you have to isolate for 10/14 days at home anyway is it?
It’s a new pandemic, the world is struggling. Yes things can/could be done better, but people are trying.